Violent clashes broke out near the Israeli embassy in London yesterday as tens of thousands marched in protest against the military action in Gaza. Shop windows were smashed and police pelted with missiles by masked youths near the embassy during the largest demonstration in Britain against the Israeli offensive.

Last night, broken glass and debris littered the scene of the disturbances on Kensington High Street, where ranks of riot police waited behind locked gates near the embassy entrance.

Earlier in the evening, a number of demonstrators attacked a branch of Starbucks, smashing its front windows and ransacking it before shattering the facade of a clothes shop. Panic rippled through the crowd as riot police advanced repeatedly with batons drawn before being later backed up by dozens of mounted police.

Officers were pelted with missiles, including shards of glass from shattered shopfronts, as stewards from the demonstration called for calm and tried to separate police from protesters.

Women and children took cover in doorways as missiles were hurled at helmeted riot police. Youths let off firecrackers and fire extinguishers and pushed over crowd control barriers. Three officers were injured, including one who was knocked unconscious, the Metropolitan police said. At least 15 people were arrested as the remaining demonstrators were corralled by riot police and held in the area for a number of hours after darkness fell.

The trouble came after tens of thousands of protesters from groups including the Stop the War Coalition marched from Speakers' Corner in Hyde Park to another demonstration at Kensington Gardens, near the embassy. Earlier, protesters had also tried to force entry to the north gate of Kensington Gardens and six climbed up an adjoining wall, smashing two lamps and setting an American flag on fire.

Protester Ahmed Mohammad, 23, said that he saw women and children hurt in police charges: "It was a peaceful protest until the riot police came. I've seen a mother and little girl pushed to the ground."

The Stop the War Coalition, which organised the demonstration, claimed that "at least" 100,000 people had braved the cold, making it "the biggest demonstration of solidarity with the Palestinian people in the history of this country". The Metropolitan police estimated the number of marchers at 20,000.

In one of several speeches delivered in Kensington Gardens, George Galloway, leftwing MP for the Respect party, called on protesters to go to shopping centres and "shut down Israel Shops" – a reference to mobile retailers who operate in malls, selling Israeli products.

At the start of the march, Speakers' Corner was turned into a sea of Palestinian flags and banners condemning Israel. The crowd, which ranged from young British Muslims to leftwing activists to students and pensioners, listened to speakers including human rights advocate Bianca Jagger, singer Annie Lennox and the Rev Garth Hewitt, canon at St George's Cathedral in Jerusalem.

• This article was amended on Tuesday 13 January 2009. We reported that George Galloway MP had called for a boycott of 'Israel's shops'. This was incorrect. He called for a boycott of Israel Shops, mobile retailers who operate in shopping malls selling Israeli goods. This has been corrected.